A deposit of carbonate rocks that once existed about 4 miles (6 kilometers) below the surface of Mars was uplifted and exposed by an ancient meteor impact, said Joseph Michalski, from PSI in Tucson, Arizona. The carbonate minerals exist along with hydrated silicate minerals of a likely hydrothermal origin.

Using data returned from NASA's Mars Reconnaissance Orbiter (MRO) spacecraft, researchers have spotted this unique mineralogy within the central peak of a crater to the southwest of a giant martian volcanic province named Syrtis Major. With infrared spectra from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), planetary geologists detected the hydrothermal minerals from their spectroscopic fingerprints.

Visible images from the High Resolution Imaging Science Experiment (HiRISE) camera aboard MRO show that the carbonates and hydrated silicate minerals occur within deformed bedrock that was exhumed by an ancient meteor impact that poked through the volcanic upper crust of Mars.

"Carbonate rocks have long been a Holy Grail of Mars exploration for several reasons," Michalski said. "One reason is because carbonates form with the ocean and within lakes on Earth, so the same could be true for ancient Mars — such deposits could indicate past seas that were once present on Mars. Another reason is because we suspect that the ancient martian atmosphere was probably denser and CO2-rich, but today the atmosphere is quite thin so we infer that the CO2 must have gone into carbonate rocks somewhere on Mars."

While this is not the first detection of carbonates on Mars, Michalski said, "This detection is significant because it shows other carbonates detected by previous workers, which were found in a fairly limited spatial extent, were not a localized phenomenon. Carbonates may have formed over a very large region of ancient Mars, but have been covered up by volcanic flows later in the history of the planet. A very exciting history of water on Mars may be simply covered up by younger lava!"

The discovery also has implications for the habitability of the martian crust.

"The presence of carbonates along with hydrothermal silicate minerals indicates that a hydrothermal system existed in the presence of CO2 deep in the martian crust," Michalski says. "Such an environment is chemically similar to the type of hydrothermal systems that exist within the ocean floor of Earth, which are capable of sustaining vast communities of organisms that have never seen the light of day."

"The cold, dry surface of Mars is a tough place to survive, even for microbes," said Michalski. "If we can identify places where habitable environments once existed at depth, protected from the harsh surface environment, it is a big step forward for astrobiological exploration of the Red Planet."